Two-phase bipolar or 4-phase unipolar step motors have been popularly used in the motion, control industry, starting first with a 3.6° step motor (100 full steps per revolution). The 3.6° stepper was very popular early in the step motor history, because it can run at a good speed. (A bigger step size with the same pulse rate provides a higher speed in terms of revolution per sec.) However, in order to satisfy constraints, the number of stator poles must be 4, which is not an efficient design.
Meanwhile, applications increasingly required better resolution than speed. As a result, a 1.8° stepper was introduced. Design constraints allow this motor to be constructed with 8 stator poles. The 8-pole design is an efficient design and easy for manufacturing. Thus, 1.8° steppers quickly became the most popular step motor.
Demand of higher resolution steppers increased in the 1980s, especially for hard disk drive (HDD) applications. As a result, first a 0.9° stepper and later a 0.45° stepper were introduced.
A key constraint of a stepper design is to satisfy the following equations:S=full step angle=360°/(Nr×Np)Nr/Nsp+(1/Np) or Nr/Nsp−(1/Np)must be integer,where Nr=Number of rotor teeth;                Nsp=Number of stator poles;        Np=Number of mechanical phases                    =3 for 3-phase unipolar motor            =4 for 2-phase bipolar motor.                        
In order to meet this general rule, for a 2-phase bipolar stepper, a 3.6° stepper must have 4 uniformed stator poles, a 1.8° stepper must have 8 uniformed stator poles, a 0.9° stepper must have 16 uniformed stator poles, and a 0.45° stepper must have 32 poles. Because 8-pole stators are easy and inexpensive to manufacture, while higher numbers of stator poles (e.g. 16 or 32) cost much more to produce, a modified 8-pole 0.9° stepper was developed by shifting the stator pole teeth to satisfy the rule. Subsequently, a modified 12-pole 0.9° stepper was also designed to accommodate the 8-pole and 16-pole designs (U.S. Pat. No. 4,910,475).
When designing the number of rotor (and stator) teeth for the motor, the preference in the industry has always been for full step angles such as 3.6°, 1.8°, 1.2°, 0.9°, 0.6°, and 0.45°, corresponding to exactly 100, 200, 300, 400, 600 and 800 steps per a complete 360° rotation. Most engineers stick to step angles of 1.8°, 0.9° or 0.45° for a 4-phase unipolar stepper or a 2-phase bipolar stepper, and step angles of 1.2° or 0.6° for a 3-phase unipolar stepper or a 3-phase bipolar motor.
Here is a table showing design parameters for the most popular stepper motors that have been used in the industry:
Standard Popular DesignS = 360/(Nr*Np)NrNpNsp(degree)16367.532363.7550362.464361.875100312 1.2200360.625443.650481.81004 8*0.9100412*0.9100416 0.92004 8*0.45200412*0.45200432 0.4550612 1.2506 9*1.21006 9*0.6100612*0.6501010*0.721001010*0.362001010*0.18*non-uniform stator teeth distribution on the pole
Demand for smaller size motors have developed more recently. It is getting harder to manufacture 1.8° steppers when the motor size becomes smaller and smaller. In addition to overall manufacturability, a small motor generally can't produce enough torque for many desired applications. Often, a gear reducer must be added to increase the torque. In such cases, speed becomes more important than the resolution.
An 8 stator pole design is still the best choice, because narrower tooth designs not only are difficult to produce, but also lose torque. For adequate magnetization of the teeth and contrast with respect to the spaces between the teeth, both the tooth width and the tooth separation must generally be a minimum of 0.5 mm. Any narrower or closer, and loss of torque would become substantial. Meanwhile, to fully utilize the effective magnetic interaction between stator and rotor, we need to maximize the number of stator teeth, while still maintaining enough space between adjacent stator poles for the winding needle to pass (in order to form the electromagnetic coils around each stator pole). Typically, the winding needle space is a minimum of 1.07 mm. The available space is a function of the stator's inner diameter (ID). Therefore, selecting a proper stator ID for the certain stepper is part of the design criteria. An 8-pole 1.8° 2-phase bipolar stepper has a minimum stator ID of 19 mm in order to accommodate 6 teeth per stator pole (48 total stator teeth) plus the requisite winding needle space. Smaller 1.8° steppers must have fewer stator teeth per pole in order to leave room to accommodate the winding needle, sacrificing torque as a result.